Multi-Stack Rolled-Up Information Carrier

ABSTRACT

The invention relates to an information carrier for scanning information by means of an optical beam having a wave-length. The information carrier comprises a central hole ( 11 ) and at least two information stacks ( 13, 14 ) rolled up around said hole. Each information stack comprises a first electrode, a second electrode and a material whose optical properties at the wavelength of the optical beam depend on a potential difference applied between the first and second electrodes.

FIELD OF THE INVENTION

The present invention relates to a multi-stack optical informationcarrier.

The present invention also relates to a scanning device for scanning amulti-stack optical information carrier.

The present invention also relates to a method of manufacturing amulti-stack optical information carrier.

BACKGROUND OF THE INVENTION

Patent U.S. Pat. No. 6,386,458 describes a data storage mediumcomprising an information carrier which is rolled up in a spiral fashionand on which written information patterns are provided which can be readoptically, due to local refractive index variations. This data storagemedium comprises a plurality of information layers, which are read fromthe inside by means of an optical beam arranged in the inside of thewinding. Such a data storage medium is compact and can, in theory,comprise a large number of information layers.

However, the number of information layers in such an information carrieris limited. First, because the luminous intensity of the optical beamdecreases with each additional layer crossed. Actually, when the opticalbeam has to pass many layers for interacting with a layer, interactiontakes also place in the layers that are not read out, reducing theintensity of the optical beam. Additionally, the local refractive indexvariations of the written information patterns in the layers that arenot read out cause refraction and scattering of the traversinglight-beam, leading to deteriorated writing and reading.

Hence, such a rolled up information carrier is not suitable formulti-layer information carriers, in particular for information carrierscomprising more than three layers.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an information carrier,which is highly compact and can comprise an increased number of layers.

To this end, the invention proposes an information carrier for scanninginformation by means of an optical beam having a wavelength, saidinformation carrier comprising a central hole and at least twoinformation stacks rolled up around said hole, wherein each informationstack comprises a first electrode, a second electrode and a materialwhose optical properties at the wavelength of the optical beam depend ona potential difference applied between the first and second electrodes.

According to the invention, the information carrier comprises aplurality of information stacks, which are rolled up around a centralhole. This makes this information carrier particularly compact.Moreover, information can be read from the inside of the informationcarrier, by means of an optical system mounted with rotation possibilityin the central hole of the information carrier. Compared with aconventional optical disc apparatus, such as a CD reader, in which theoptical disc rotates during scanning, the rotational speed of theoptical system can be higher than the rotational speed of a conventionaloptical disc, thus increasing the data transfer rate.

Furthermore, the information layers comprise a material, whose opticalproperties can be switched by application of a potential difference.Hence, by application of suitable potential differences to the stacks,it is possible to scan one layer having optical properties suitable forinteracting with the optical beam, whereas the optical properties of theother layers are chosen such that the interactions between thesenon-addressed layers and the optical beam are reduced. As a consequence,the number of layers can be increased.

In a first embodiment of the invention, the information carriercomprises an electrolyte layer between the first and the secondelectrode, the first electrode being an information layer comprising anelectrochromic material, the second electrode being a counter electrode.

Advantageously, an information layer serves as counter electrode foranother information layer. This reduces the number of layers of thestacks. Hence, the information carrier is less bulky, and themanufacturing process of the information carrier is simplified.

Preferably, the electrochromic material has an ability to take up orrelease electrons, which ability can be locally reduced by means of theoptical beam in order to write information on the information layer.Information can thus be written on the information carrier by a user.

With particular preference, the electrolyte layer has atemperature-dependent mobility threshold. According to this embodiment,information can be written by a user, then erased and rewritten on theinformation carrier.

Advantageously, the information layer further comprises a thermochromicmaterial having temperature-dependent optical properties at thewavelength of the optical beam. When the electrolyte layer has atemperature-dependent mobility threshold, allowing rewriting ofinformation on the information carrier, the layers of the informationcarrier all have the same optical properties during read-out of theinformation. Hence, the non-addressed layers interact with the opticalbeam, which reduces the possible number of layers. The use of athermochromic material reduces the interaction between the optical beamand the layers, because the thermochromic material improves theinteraction between the optical beam and the addressed layer.

Preferably, an information stack further comprises a photoconductivelayer for allowing a transfer of electrons in the information layer whenilluminated at the wavelength of the optical beam. When the electrolytelayer has a temperature-dependent mobility threshold, allowing thewriting of marks on the information carrier, the diffusion of heat inthe electrolyte layer during writing of information makes the marksrelatively large. The use of a photoconductive layer reduces the size ofthe written marks, thus increasing the data capacity of the informationcarrier.

The information layers may further comprise a fluorescent material.Read-out of information is performed by detection of light emitted byfluorescence. As a consequence, the number of layers may be increasedeven more.

Advantageously, the fluorescent material has an ability to emit light byfluorescence, which ability can be locally reduced by means of theoptical beam in order to write information on the information layer.According to this embodiment, information can be written by a user onthe information carrier.

In a second embodiment of the invention, the information carriercomprises an information layer between the first and second electrodes,wherein the information layer comprises molecules which can be rotatedwhen a suitable potential difference is applied between the first andsecond electrodes.

Advantageously, said molecules are liquid crystal molecules which can berotated when subjected to an electric field created by the potentialdifference applied between the first and second electrodes.

Preferably, said molecules comprise a charged substituent which can berotated when subjected to a current created by the potential differenceapplied between the first and second electrodes.

With particular preference, the first electrode has an electricalconductance which can be locally reduced by means of an optical beam inorder to write information on the information layer. Information canthus be written on the information carrier by a user.

Preferably, the information stack further comprises a thermal insulationlayer between the first electrode and the information layer. In thiscase, writing of information without degrading the information layer ispossible, even if the information layer has a decomposition temperaturewhich is lower than or equal to the temperature at which the electricalconductance of the first electrode is reduced. If this insulation layeris an electrically insulating layer, the embodiment based on moleculesthat rotate under the influence of an electric field may be used. If anelectrically conducting layer is used, the embodiment based on moleculesthat rotate under the influence of an electrical current may also beused.

Advantageously, the information layer can be locally degraded by meansof an optical beam in order to write information on the informationlayer. The information layer may be for example annealed, altered,molten, fixed or photochemically deteriorated by means of the opticalbeam in order to write information, such that a further orientationchange of the molecules of the information layer is no longer possible.The degraded parts of the layer remain essentially transparent, whateverthe potential difference applied between the first and secondelectrodes. According to this embodiment, information can be written onthe information carrier by a user in that certain areas of theinformation stack are disabled from changing their optical properties.

Preferably, the information layer comprises a matrix comprising twotypes of surface-charged colloidal particles, one with negative chargeand one with positive charge, said surface-charged colloidal particlescomprising liquid crystal molecules, said matrix having a viscositywhich can be locally reduced by means of an optical beam in order towrite information on the information layer. According to thisembodiment, information can be written by a user, then erased andrewritten on the information carrier.

The invention also relates to an optical scanning device for scanning aninformation carrier by means of an optical beam having a wavelength,said information carrier comprising a central hole and at least twoinformation stacks rolled up around said hole, wherein each informationstack comprises a first electrode, a second electrode and a materialwhose optical properties at the wavelength of the optical beam depend ona potential difference applied between the first and second electrodes,said optical scanning device comprising means for receiving saidinformation carrier, means for generating the optical beam, means forapplying a potential difference between the information layer and thecounter electrode of an information stack, means for focusing saidoptical beam on an information layer, said focusing means being mountedwith rotation possibility inside said receiving means.

Advantageously, said focusing means are mounted with translationpossibility inside said receiving means. The information carrier maythus be completely fixed in the optical scanning device. The powerneeded for translating the focusing means may be lower than the powerneeded for translating the information carrier. The power needed forrotating the focusing means is lower than the power which would berequired to rotate the information carrier.

The invention also relates to a method of manufacturing an informationcarrier, said method comprising the steps of manufacturing aninformation strip comprising at least one electrode and a material whoseoptical properties at the wavelength of the optical beam depend on apotential difference applied between two electrodes, winding saidinformation strip around a transparent hollow element and cutting theelectrode at each turn of the winding step.

Advantageously, this method further comprises a step of writinginformation on the information strip. A ROM information carrier can thusbe obtained in a continuous process. Furthermore, this writing step mayalso be used to make a pre-grooved strip as required for write-once(WORM) and rewritable (RW) information carriers.

These and other aspects of the invention will be apparent from and willbe elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 shows an information carrier in accordance with the invention;

FIG. 2 illustrates a method a manufacturing an information carrier inaccordance with the invention;

FIG. 3 a is a detailed view of a first ROM information carrier inaccordance with a first embodiment of the invention and FIG. 3 b shows acomplete view of this first ROM information carrier;

FIG. 4 shows a detailed view of a second ROM information carrier inaccordance with a first embodiment of the invention;

FIGS. 5 a, 5 b, and 5 c show a third, a fourth and a fifth informationcarrier in accordance with a first embodiment of the invention;

FIGS. 6 a, 6 b and 6 c show a first, a second and a third ROMinformation carrier in accordance with an advantageous embodiment of theinvention;

FIG. 7 shows a first ROM information carrier in accordance with a secondembodiment of the invention;

FIGS. 8 a and 8 b show a second and a third ROM information carrier inaccordance with a second embodiment of the invention;

FIG. 9 shows a WORM information carrier in accordance with a firstembodiment of the invention;

FIGS. 10 a, 10 b, 10 c and 10 d show a first, a second, a third and afourth WORM information carrier in accordance with a second embodimentof the invention;

FIGS. 11 a and 11 b show a first and a second RW information carrier inaccordance with a first embodiment of the invention;

FIG. 12 shows a structure of an unwritten information layer in a RWinformation carrier in accordance with a second embodiment of theinvention;

FIG. 13 shows a structure of a written information layer in a RWinformation carrier in accordance with a second embodiment of theinvention; and

FIG. 14 shows an optical device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

An information carrier in accordance with the invention is depicted inFIG. 1. This information carrier comprises a transparent hollow element12 comprising a central hole 11, a first, a second, a third and a fourthinformation stack 13, 14, 15 and 16, and a protective cover 17. Theinformation carrier further comprises contacts, such as contacts 16 aand 16 b.

It should be noted that an information stack may be a combination of twoinformation stacks of the information carrier represented in FIG. 1, aswill be described in more detail in FIG. 6 a for example. For example,the first and second information stacks 13 and 14 may form aninformation stack.

In the example of FIG. 1, the information carrier comprises two contactsper information stack. For example, contact 16 a is connected to a firstelectrode of the fourth information stack 16 and contact 16 b isconnected to a second electrode of the fourth information stack 16. Ifan information stack is a combination between two information stacksshown in FIG. 1, the information carrier may comprise only one contactper information stack shown in FIG. 1.

The information carrier of FIG. 1 is intended to be scanned by anoptical beam, which has a wavelength 1. In the example of FIG. 1, eachinformation stack comprises a first electrode, a second electrode and amaterial whose optical properties at the wavelength of the optical beamdepend on a potential difference applied between the first and secondelectrodes. The hollow element 12 is chosen so as to be transparent atthe wavelength 1. By application of a suitable potential differencesbetween the suitable contacts, it is possible to scan information in aninformation stack, which scanning is not perturbed by the presence ofthe other information stacks. Actually, depending on the potentialdifference applied to an information stack, the material of thisinformation stack may be absorbent or transparent at the wavelength 1.Hence, in order to scan the fourth information stack 16 for example, thefirst, second and third information stacks 13, 14 and 15, can be madetransparent at the wavelength 1, whereas the material of the fourthinformation stack 16 is made absorbent at the wavelength 1. Ifinformation has been written in this fourth information stack 16 bymeans of said material, information can be read from the fourthinformation stack 16, without this read-out being perturbed by thefirst, second and third information stacks 13, 14 and 15.

It should be noted that an information carrier in accordance with theinvention may comprise more than four information stacks. Actually, asthe scanning of an information stack is not perturbed by the otherinformation stacks, such an information carrier may comprise arelatively high number of information stacks. For example, aninformation carrier in accordance with the invention may comprise 10, 20or up to 100 or more information stacks.

It should also be noted that the thickness of the information stacksshown in FIG. 1 does not correspond to reality. Actually, the thicknessof an information stack may be several micrometers, typically 40micrometers, preferably less than 10 micrometers, most preferably lessthan 1 micrometer, whereas the diameter of the central hole may be aboutone centimetre. Hence, such an information carrier is relativelycompact, even if the number of information stacks is high.

It should also be noted that information carriers in accordance with theinvention may have various shapes. For example, the information carriermay be elliptically shaped, or have a parallelepipedic shape. Theoverall shape of the data storage medium may also be a hollow rectangle.The readout of data then preferably follows a meandering or zig-zag pathbut a helical path is also possible.

FIG. 2 illustrates a method of manufacturing an information carrier inaccordance with the invention. FIG. 2 corresponds to a top view of theinformation carrier of FIG. 1, during its manufacturing process. Themanufacture consists in making an information strip comprising at leastone electrode, such as electrode 261 and a material whose opticalproperties at the wavelength of the optical beam depend on a potentialdifference applied between two electrodes. The information strip furthercomprises an additional layer, such as additional layer 26, which atleast comprises an adhesive against unwanted unwinding.

The nature of the information strip depends on the material used, whoseoptical properties can be switched by means of a potential difference,as well as on the design of the information stacks. Many examples ofinformation stacks and structures of the information strip are describedin more details with reference to the following Figures.

Once the information strip has been manufactured, it is rolled up aroundthe transparent hollow element 12. In the example of FIG. 2, theinformation strip is rolled up in a spiral fashion. At each turn, theelectrode of the information strip is cut, so that a plurality ofelectrodes are obtained, which are electrically insulated from eachother. In this example, the information carrier comprises fiveelectrodes 211, 221, 231, 241 and 251. The information strip maycomprise other electrodes, which are also cut at each turn of thewinding.

The electrodes may also be cut before winding, so as to obtain aplurality of electrodes in the information strip before winding. In thiscase, the length of an electrode in the information strip corresponds tothe length of a turn of the information carrier.

Other layers of the information strip, such as the additional layer 26,may also be cut before or during winding. In the example of FIG. 2, theinformation carrier comprises five additional layers 21, 22, 23, 24 and25. In this example, the information carrier comprises five informationstacks, each comprising an electrode, such as electrode 211, and anadditional layer, such as additional layer 21.

At the end of the manufacturing process, a protective layer is rolled upor deposited around the information stacks. Contacts are then added onthe top surface of the information carrier, which contacts areelectrically connected to the associated electrodes of the informationstacks. These contacts may also be added before or during winding. Theinformation carrier may comprise further contacts on its bottom surface.This allows the stacking together of a plurality of information carriersin accordance with the invention, in that the contacts of the topsurface of one information carrier are aligned with the contacts of thebottom surface of another information carrier.

It is important to note that the information strip may be manufacturedduring winding. For example, in a case where information is written onthe information strip by embossing, the information strip may beembossed while an already embossed part of the information strip isbeing rolled up. As a consequence, the manufacturing process iscontinuous, thus reducing the time of manufacture of an informationcarrier in accordance with the invention. Instead of embossing theinformation, printing, pressing or other methods of data replicationsuch as burning or photo-chemical deterioration, are possible.

FIG. 3 a is a detailed view of a first ROM (Read Only Memory)information carrier in accordance with a first embodiment of theinvention. FIG. 3 a corresponds to a view in the plane P of FIG. 1,where only two information stacks, such as the first and secondinformation stacks 13 and 14 of FIG. 1, are shown.

This information carrier comprises a first information layer 31, a firstelectrolyte layer 32, a first counter electrode 33, a spacer layer 34, asecond information layer 35, a second electrolyte layer 36 and a secondcounter electrode 37. The first information layer 31, the firstelectrolyte layer 32 and the first counter electrode 33 form a firstinformation stack, such as the first information stack 13, while thesecond information layer 35, the second electrolyte layer 36 and thesecond counter electrode 37 form a second information stack, such as thesecond information stack 14. The two information stacks are separated bythe spacer layer 34. The spacer layer 34 is comprises an adhesivematerial.

In this example, the information strip rolled up so as to obtain thisinformation carrier comprises an information layer, an electrolytelayer, a counter electrode and a spacer layer.

The first information layer 31 is connected to a first contact, thefirst counter electrode 33 to a second contact, the second informationlayer 35 to a third contact and the second counter electrode 37 to afourth contact.

This information carrier is a ROM (Read Only Memory) informationcarrier, which means that a user cannot record information on thiscarrier. The information is recorded during the manufacturing processand cannot be erased. The information layers 31 and 35 comprise pits andlands, which are obtained by means of conventional techniques, such asembossing and printing.

This information carrier is intended to be scanned by an optical beam,which has a wavelength 1. The first and second electrolyte layers 32 and36, the first and second counter electrodes 33 and 37 as well as thespacer layer 34, are chosen to be transparent at the wavelength 1, or atleast to have a very small absorption at this wavelength, in order notto interact with the optical beam.

The first and second information layers 31 and 35 comprise anelectrochromic material. An electrochromic material is a material havingoptical properties, which can change as a result of electron uptake orloss. Electrochromic materials are known to those skilled in the art.For example, the publication “Electrochromism: Fundamentals andApplications”, by Paul M. S. Monk et al. and published in 1995,describes the properties of electrochromic materials. Preferably, theelectrochromic materials used in an information carrier in accordancewith the invention are thiophene derivatives, such aspoly(3,4-ethylenedioxythiophene), also called PEDT or PEDOT anddescribed, for example, in “Poly(3,4-ethylenedioxythiophene) and ItsDerivatives: Past, Present and Future”, by L. Bert Goenendaal et al.,published in Advanced Materials 2000, 12, No. 7.

In the example of FIG. 3, the electrochromic material of the first andsecond information layers 31 and 35 is the same, and has a reduced stateand an oxidized state. The electrochromic material is chosen to have ahigh absorption and reflection at the wavelength 1 when it is in itsreduced state, and a low absorption and reflection at the wavelength 1when it is in its oxidized state. Of course, another electrochromicmaterial could be used, which has a high absorption and reflection atthe wavelength 1 when it is in its oxidized state, and a low absorptionand reflection at the wavelength 1 when it is in its reduced state.

When the first information layer 31 is scanned for reading informationfrom this first information layer 31, a potential difference V1 isapplied between the first information layer 31 and the first counterelectrode 33, i.e. between the first and second contacts, the firstinformation layer 31 being at a higher potential than the first counterelectrode 33. A current then flows from the first information layer 31to the first counter electrode 33, whereas electrons are transportedfrom the first counter electrode 33 to the first information layer 31.Electrons are absorbed by the electrochromic material, which becomesreduced. For reasons of electrical neutrality, positive ions from thefirst electrolyte layer 32 are absorbed by the first information layer31 or negative ions are expelled by the first information layer 31, andnegative ions from the first electrolyte 32 are absorbed by the firstcounter electrode 33 or positive ions are expelled by the first counterelectrode 33. Hence, the first counter electrode is an ion-accepting anddonating electrode. The potential difference V1 is chosen such that,when applied, the absorption and reflection of the first informationlayer 31 become relatively high at the wavelength 1. The requiredpotential difference V1 depends on the wavelength 1, the electrochromicmaterial, the electrolyte, the counter electrode, and an optionaladditional electrode in the information stack.

Then, once the absorption and reflection of the first information layer31 have become high, the potential difference can be cut. Actually, theused electrochromic materials display bistability, which means thattheir optical properties persist when no potential difference isapplied. As the absorption and reflection of the first information layer31 are high, information can be read from this information layer byconventional read-out techniques, such as the phase difference read-outprinciple used, for example, for read-out of CD-ROM.

The electrolyte layer of an information stack comprises an electrolyte,which should be able to provide ions to the information layer and thecounter electrode of this information stack. Preferably, solid orelastomeric polymeric electrolytes are used in an information carrier inaccordance with the invention. These electrolytes consist of polymerscomprising ion-labile groups, or consist of polymers with dissolvedsalts. Examples of polymers with dissolved salts are crosslinkedpolyethers, polyethylene oxide, polyvinyl alcohol or polymethylmethacrylate, with salts such as lithium chlorate, triflic acid orphosphoric acid.

Once the information of the first information layer 31 has been read,the second information layer 35 is scanned. First, the first informationlayer 31 is made transparent in that a potential difference −V1 isapplied between the first information layer 31 and the first counterelectrode 33, which is a reverse potential difference compared with V1.As a consequence, the electrochromic material of the first informationlayer 31 becomes oxidized, in which state it has a low absorption andreflection at the wavelength 1. The potential difference −V1 can then becut, because of the bistability of the electrochromic material of thefirst information layer 31. Then, the second information layer 35 ismade absorbent, in that a potential difference V2 is applied between thesecond information layer 35 and the second counter electrode 37. In thisexample, V2 is equal to V1, because the first and second informationstacks comprise the same electrochromic material. If differentelectrochromic materials are used in the first and second informationlayers 31 and 35, V2 may differ from V1. Once the second informationlayer 35 has become absorbent, the potential difference V2 is cut,because the used electrochromic material is bistable.

Once the absorption of the second information layer 35 is high,information can be read from this information layer. The firstinformation layer 31 does not perturb the read-out of information,because the first information layer 31 has been made transparent. As aconsequence, it is possible to address only one information layer, whilethe rest of the information carrier is transparent or has a lowabsorption and reflection. The desired layer is addressed in that thesuitable potential differences are applied between the informationlayers and the counter electrodes of the different information stacks.

An information carrier in accordance with the invention, comprising theabovementioned layers, may be manufactured by conventional techniques,such as embossing, moulding, photolithographic techniques, micro-contactprinting or vapour deposition.

An information carrier in accordance with the invention may comprisemore than two information stacks. For example, an information carrier inaccordance with the invention may comprise 10, 20 or up to 100 or moreinformation stacks. For example, an information carrier in accordancewith the invention, which comprises 4 information stacks, is depicted inFIG. 3 b. FIG. 3 b corresponds to a complete view in the plane P of theinformation carrier of FIG. 1.

FIG. 4 is a detailed view of a second ROM information carrier inaccordance with a first embodiment of the invention. FIG. 4 alsocorresponds to a view in the plane P of FIG. 1, where only twoinformation stacks, such as the first and second information stacks 13and 14 of FIG. 1, are shown.

Such an information carrier comprises a first information layer 41, afirst electrolyte layer 42, a first counter electrode 43, a spacer layer44, a second information layer 45, a second electrolyte layer 46 and asecond counter electrode 47. The first information layer 41, the firstelectrolyte layer 42 and the first counter electrode 43 form a firstinformation stack, the second information layer 45, the secondelectrolyte layer 46 and the second counter electrode 47 form a secondinformation stack. The two information stacks are separated by thespacer layer 44.

An information layer comprises pits and lands, the pits being filled bya fluorescent material, the lands comprising an electrochromic material.For example, the first information layer 41 comprises lands 410, whichcomprise an electrochromic material, and pits 411, which comprise afluorescent material.

The information strip rolled up so as to obtain this information carriercomprises an information layer, an electrolyte layer, a counterelectrode and a spacer layer. Such an information strip is manufacturedby conventional techniques, such as those described in patent WO98/50914.

For example, an electrolyte layer is deposited on a counter electrode.Then, a layer comprising the electrochromic material is deposited on theelectrolyte layer. A stamper comprising a plurality of convexities isapplied to this layer comprising the electrochromic material. Thisresults in a pattern on the surface of this layer, said pattern matchingthe convexities of the stamper. Then, a layer comprising the fluorescentmaterial is deposited on the surface of the patterned layer. This layercomprising the fluorescent material is chosen so as to have goodadhesion properties to the patterned layer. A portion of this layerpenetrates into the pits of the patterned layer and another portionremains on the surface of the lands of the patterned layer. Said otherportion is then eliminated by means of a suitable solvent. Theinformation layer is thus obtained, which comprises lands comprising theelectrochromic material, and pits filled with the fluorescent material.These pits filled with the fluorescent material are fluorescent cells,which comprise the information recorded on the information layer. Suchan information strip may also be manufactured by means of an injectionmolding technique, as described in WO 98/50914.

This information carrier is intended to be scanned by an optical beam,which has a wavelength 11. The first and second electrolyte layers 42and 46, the first and second counter electrodes 43 and 47, the spacerlayer 44, as well as the fluorescent material are chosen so as to betransparent at the wavelength 11.

When the first information layer 41 is scanned for reading informationfrom this first information layer 41, a potential difference V1 isapplied between the first information layer 41 and the first counterelectrode 43. Electrons are absorbed by the electrochromic material ofthe first information layer 41, which becomes reduced. The potentialdifference V1 is chosen such that, when applied, the absorption andreflection of the electrochromic material of the first information layer41 become relatively high at the wavelength 11.

As the absorption and reflection of the first information layer 41 arehigh, this first information layer 41 absorbs energy from the opticalbeam focused on this information layer. When the optical beam is focusedon a pit of the first information layer 41, the absorbed energy isconverted into a fluorescence signal by the fluorescent materialcomprised in this pit. This fluorescence signal is then detected byconventional techniques. Hence, information is read from the firstinformation layer 41. Examples of such fluorescent materials arequinoline, acridine, indole, coumarin derivatives, such as2,3,5,6-1H,4H-tetrahydro-9-acetylquinolizino-[9,9a,1-gh]-coumarin and3-(2′-N-methylbenzimidazolyl)-7-N,N-diethylaminocoumarin, andpyrromethene derivatives. These fluorescent materials may be applied assuch, or be dispersed in a supporting matrix material, for example of apolymeric nature, with the optional aid of complexation or adsorption toa binder.

Once the information of the first information layer 41 has been read,the second information layer is scanned. First, the first informationlayer 41 is made transparent in that a potential difference −V1 isapplied between the first information layer 41 and the first counterelectrode 43. As a consequence, the electrochromic material of the firstinformation layer 41 becomes oxidized, in which state it has a lowabsorption at the wavelength 11. Then, the second information layer 45is made absorbent, in that a potential difference V2 is applied betweenthe second information layer 45 and the second counter electrode 47. Inthis example, V2 is equal to V1.

Once the absorption of the second information layer 45 is high, thissecond information layer 45 can absorb energy from the optical beamfocused on this second information layer 45. When the optical beam isfocused on a pit of the second information layer 45, the absorbed energyis converted into a fluorescence signal by the fluorescent materialcomprised in this pit. This fluorescence signal is then detected, andinformation is thus read from the second information layer 45.

Due to the so-called Stokes-shift, the fluorescence signal has awavelength 12. The fluorescent material is chosen so as to betransparent at the wavelength 12, so that the detected fluorescencesignal is not perturbed by the fluorescent material of the firstinformation layer 41.

The first information layer 41 does not perturb the read-out ofinformation recorded on the second information layer 45, because theelectrochromic material of the first information layer 41 has been madetransparent at the wavelength 11, as explained hereinbefore. Hence, theoptical beam at wavelength 11 traversing the first information layer 41does not interact with the lands of the first information layer 41, anddoes not interact with the pits of the first information layer 41either, because the fluorescent material is chosen to be transparent atthe wavelength 11. Moreover, the electrochromic material of the firstinformation layer 41 is chosen to be transparent at the wavelength 12,so that it does not interact with the fluorescence signal at wavelength12.

As a consequence, it is possible to address only one information layer,while the rest of the information carrier is transparent or has a lowabsorption at the wavelength 11 and at the wavelength 12. The desiredlayer is addressed in that the suitable potential differences areapplied between the information layers and the counter electrodes of therespective information stacks.

FIGS. 5 a, 5 b and 5 c show a second, a third and a fourth ROMinformation carrier in accordance with a first embodiment of theinvention. In these Figures, numbers identical to numbers of FIG. 3 astand for the same entities. These information carriers comprise a firstand a second information stack. The first information stack comprises afirst transparent electrode 51 and a second transparent electrode 52.The second information stack comprises a third transparent electrode 53and a fourth transparent electrode 54. The first, second, third andfourth transparent electrodes 51 to 54 are chosen to be transparent atthe wavelength 1.

The information strip rolled up so as to obtain the information carrierof FIG. 2 a comprises a first transparent electrode, a counterelectrode, an electrolyte layer, an information layer comprising anelectrochromic material, a second transparent electrode and a spacerlayer. The information layer of the information strip is patterned byconventional techniques, such as embossing. The second transparentelectrode is deposited on the information layer by conventionaltechniques, such as vapour deposition.

In order to switch the first information layer 31 from a transparentstate to an absorbent state at the wavelength 1, a suitable potentialdifference is applied between the first and second transparentelectrodes 51 and 52. This potential difference depends, inter alia, onthe nature of the first and second transparent electrodes 51 and 52.Examples of materials which may be used for the first and secondtransparent electrodes 51 and 52 are ITO (Indium Tin Oxide), PPV(poly(phenylenevinylene)), PEDOT (poly(3,4-ethylenedioxythiophene) andother polythiophene derivatives. In order to switch the firstinformation layer 31 from an absorbent state to a transparent state atthe wavelength 1, a reverse potential difference is applied between thefirst and second transparent electrodes 51 and 52. This description alsoapplies to the second information stack.

The information strip is rolled up so as to obtain the informationcarrier of FIG. 5 b comprises a first transparent electrode, a counterelectrode, an electrolyte layer, an information layer comprising anelectrochromic material, a second transparent electrode and a spacerlayer. The electrolyte layer is patterned, and the information layer,comprising electrochromic material, is deposited on the lands of theelectrolyte layer, by conventional techniques, such as offset printing.The second electrode is deposited on the information layer. Theinformation layers 31 and 35 are continuous layers, as the onlydiscontinuities are caused by isolated pits. The potential differencesare applied between the first and second electrodes 51 and 52, and thethird and fourth electrodes 53 and 54, respectively.

The information strip rolled up so as to obtain the information carrierof FIG. 5 c comprises a first transparent electrode, a counterelectrode, an electrolyte layer, an information layer comprising anelectrochromic material, a second transparent electrode and a spacerlayer. The electrolyte layer is patterned, and the information layer,comprising electrochromic material, is deposited on the lands of theelectrolyte layer, by conventional techniques, such as vapourdeposition. The second transparent electrode is deposited on theinformation layer. The potential differences are applied between thefirst and second transparent electrodes 51 and 52, and the third andfourth transparent electrodes 53 and 54, respectively.

It should be noted that such additional transparent electrodes may beused in an information carrier in which the information layers comprisean electrochromic material and a fluorescent material, such as theinformation carrier of FIG. 4.

FIG. 6 a shows a first ROM information carrier, wherein an informationlayer serves as counter electrode for another information layer. Thisinformation carrier comprises a first, a second and a third informationlayer 601, 603 and 605, and a first and a second electrolyte layer 602and 604.

The information strip rolled up so as to obtain the information carrierof FIG. 6 a comprises an information layer and an electrolyte layer. Theelectrolyte layer comprises an adhesive material.

The first information layer 601, the first electrolyte layer 602 and thesecond information layer 603 form a first information stack. The secondinformation layer 603, the second electrolyte layer 604 and the thirdinformation layer 605 form a second information stack. The first andsecond information stacks thus have two information layers and twocounter electrodes. As a consequence, an information stack does notcorrespond to a part of the information strip corresponding to a singleturn of the winding process, but is the combination of two parts of theinformation strip each corresponding to a single turn of the windingprocess. This means that the first information stack of FIG. 6 a is acombination of the first and second information stacks 13 and 14 of FIG.1, and the second information stack of FIG. 6 a is a combination of thesecond and third information stacks 14 and 15 of FIG. 1.

In the first information stack of FIG. 6 a, the second information layer603 serves as counter electrode for the first information layer 601, andthe first information layer 601 serves as counter electrode for thesecond information layer 603. Actually, the first and second informationlayers 601 and 603 comprise electrochromic materials, and are thusion-accepting and donating electrodes. In the second information stack,the third information layer 605 serves as counter electrode for thesecond information layer 603, and the second information layer 603serves as counter electrode for the third information layer 605.

In order to address the first information layer 601, the firstinformation layer 601 is made absorbent in that a suitable potentialdifference V1 is applied between the first information layer 601 and thesecond information layer 603. Then, in order to address the secondinformation layer 603, the first information layer 601 is madetransparent in that a reverse potential difference −V1 is appliedbetween the first information layer 601 and the second information layer603. As a consequence, the second information layer 603 becomes reduced,and hence becomes absorbent at the wavelength 1. Hence, the secondinformation layer 603 is addressed and can be scanned.

In order to address the third information layer 605, a potentialdifference V2 is applied between the second information layer 603 andthe third information layer 605. This potential difference V2 is equalto −V1, as the electrochromic materials in the information layers 601,603 and 605 are the same. The third information layer 605 is reduced andbecomes absorbent at the wavelength 1, and the second information layer603 is oxidized and becomes transparent at the wavelength 1. As aconsequence, only the third information layer 605 is absorbent at thewavelength 1, so that the first and second information layers 601 and603 do not perturb the scanning of the third information layer 605.

FIG. 6 b shows a second ROM information carrier, wherein an informationlayer serves as counter electrode for another information layer. Thisinformation carrier comprises a first, a second, a third and a fourthinformation layer 601, 603, 605 and 607, a spacer layer 604, a first anda second electrolyte layer 602 and 606, and a first, a second, a thirdand a fourth transparent electrode 611 to 614. The first transparentelectrode 611, the first information layer 601, the first electrolytelayer 602, the second information layer 603 and the second transparentelectrode 612 form a first information stack. The third transparentelectrode 613, the third information layer 605, the second electrolytelayer 606, the fourth information layer 607 and the fourth transparentelectrode 614 form a second information stack. The two informationstacks are separated by the spacer layer 604.

The information strip rolled up so as to obtain the information carrierof FIG. 6 b comprises a first transparent electrode, a first informationlayer, an electrolyte layer, a second information layer, a secondtransparent electrode and a spacer layer. In this case, an informationstack corresponds to a part of the information strip corresponding to asingle turn of the winding process.

In order to address the first information layer 601, the firstinformation layer 601 is made absorbent in that a suitable potentialdifference V1 is applied between the first transparent electrode 611 andthe second transparent electrode 612. Then, in order to address thesecond information layer 603, the first information layer 601 is madetransparent in that a reverse potential difference −V1 is appliedbetween the first transparent electrode 611 and the second transparentelectrode 612. As a consequence, the second information layer 603becomes absorbent at the wavelength 1. Hence, the second informationlayer 603 is addressed and can be scanned.

Then, in order to address the third information layer 605, the secondinformation layer 603 has to be made transparent, so that the scanningof the third information layer 605 is not perturbed by the secondinformation layer 603. This cannot be done by application of a potentialdifference V1 between the first transparent electrode 611 and the secondtransparent electrode 612, because the first information layer 601 wouldbecome absorbent at the wavelength 1, thus perturbing the scanning ofthe third information layer 605. As a consequence, a potentialdifference different from V1 is applied between the first transparentelectrode 611 and the second transparent electrode 612, at whichpotential difference the first information layer 601 and the secondinformation layer 603 are both transparent. This is possible, becausethe absorption of certain electrochromic materials depends on theapplied potential difference, as explained, for example, in“Electrochromism: Fundamentals and Applications”, page 145. For example,a potential difference V1/2 may be applied. The potential difference tobe applied in order to make the first and second information layers 601and 603 transparent depends, inter alia, on the electrochromic materialused.

The third information layer 605 is then addressed by application of apotential difference V2 between the third transparent electrode 613 andthe fourth transparent electrode 614. In this example, V2 is equal toV1, because the electrochromic materials used in the information layersare the same. Then, in order to address the fourth information layer607, a reverse potential difference −V2 is applied between the thirdtransparent electrode 613 and the fourth transparent electrode 614.

FIG. 6 c shows a third ROM information carrier, wherein an informationlayer serves as counter electrode for another information layer. Thisinformation carrier comprises a first, a second and a third informationlayer

601, 603 and 605, a first and a second electrolyte layer 602 and 604,and a first, a second, a third, a fourth, a fifth and a sixthtransparent electrode 621 to 626. The first transparent electrode 621,the first information layer 601, the first electrolyte layer 602, thesecond information layer 603 and the fourth electrode 624 form a firstinformation stack. The third electrode 623, the second information layer603, the second electrolyte layer 604, the third information layer 605and the sixth electrode 626 form a second information stack. In thisinformation carrier, the six electrodes 621 to 626 are porous, whichmeans that ions from the electrolytes 602 and 604 can pass through theseelectrodes 621 to 626.

The information strip which is rolled up so as to obtain the informationcarrier of FIG. 6 c comprises a first transparent electrode, aninformation layer, a second transparent electrode and an electrolytelayer. The electrolyte layer comprises an adhesive material. In thiscase, an information stack does not correspond to a part of theinformation strip corresponding to a single turn of the winding process,but is the combination of two parts of the information strip eachcorresponding to a single turn of the winding process.

In order to address the first information layer 601, the firstinformation layer 601 is made absorbent in that a suitable potentialdifference V1 is applied between the first transparent electrode 621 andthe fourth transparent electrode 624. As the second and thirdtransparent electrodes 622 and 623 are porous, ions can flow between thefirst and second information layers 601 and 603, so that theelectrochemical process can be performed.

Then, in order to address the second information layer 603, the firstinformation layer is made transparent in that a reverse potentialdifference −V1 is applied between the first transparent electrode 621and the fourth transparent electrode 624. As a consequence, the secondinformation layer 603 becomes reduced, and hence becomes absorbent atthe wavelength 1. Hence, the second information layer 603 is addressedand can be scanned.

In order to address the third information layer 605, a potentialdifference V2 is applied between the third transparent electrode 623 andthe sixth electrode 626. This potential difference V2 is equal to −V1,as the electrochromic materials in the information layers 601, 603 and605 are the same. The third information layer 605 is reduced and becomesabsorbent at the wavelength 1, and the second information layer 603 isoxidized and becomes transparent at the wavelength 1. As a consequence,only the third information layer 605 is absorbent at the wavelength 1,so that the first and second information layers 601 and 603 do notperturb the scanning of the third information layer 605.

FIG. 7 shows a first ROM information carrier in accordance with a secondembodiment of the invention. Such an information carrier comprises afirst, a second, a third and a fourth transparent electrode 71, 73, 75and 77, a first and a second information layer 72 and 76 and a spacerlayer 74. The first transparent electrode 71, the first informationlayer 72 and the second transparent electrode 73 form a firstinformation stack, the third transparent electrode 75, the secondinformation layer 76 and the fourth transparent electrode 77 form asecond information stack. The two information stacks are separated bythe spacer layer 74.

This information carrier is intended to be scanned by an optical beam,which has a wavelength 1. The first, second, third and fourthtransparent electrodes 71, 73, 75 and 77 as well as the spacer layer 74,are chosen so as to be transparent at the wavelength 1.

An information layer of an information stack comprises molecules whichcan be rotated with respect to their initial orientation when a suitablepotential difference is applied between the first and second electrodes.A DC voltage may be used to accomplish this, but preferably an ACvoltage is used.

The information strip rolled up so as to obtain the information carrierof FIG. 7 comprises a first transparent electrode, an information layer,a second transparent electrode and a spacer layer. The information layeris patterned by conventional techniques such as embossing and printing.

Molecules having an ability to turn towards a given direction when apotential difference is applied between transparent electrodes are, forexample, liquid crystal molecules. Such liquid crystal molecules aredescribed, for example, in “Handbook of Liquid Crystal Research”, byPeter J. Collings, Jay S. Patel, Oxford University Press, New York,1997. For example, a suitable potential difference applied between thefirst and second transparent electrodes 71 and 73 creates an electricfield, which has a direction substantially orthogonal to the first andsecond transparent electrodes 71 and 73. When subjected to this electricfield, the liquid crystal molecules of the first information layer 72will turn towards the direction of the electric field.

This is true when liquid crystal molecules having a positive dielectricanisotropy are used. Alternatively, liquid crystal molecules having anegative dielectric anisotropy may be used in accordance with theinvention. In this case, the liquid crystal molecules of the firstinformation layer 72 will turn towards a direction that is perpendicularto the direction of the electric field. The following descriptionapplies to liquid crystal molecules having a positive dielectricanisotropy.

Furthermore, an information layer may comprise a single type of liquidcrystal molecules, or a mixture of two or more types of liquid crystalmolecules. Moreover, an information layer may exhibit one or moretemperature-dependent or concentration-dependent liquid crystal phases,such as a nematic phase, smectic phase, chiral nematic phase,ferroelectric phase or discotic phase.

Furthermore, an information layer may incorporate other components. Forinstance, the liquid crystal molecules may be incorporated within anisotropic or anisotropic network, as described, for example, in “Liquidcrystals in complex geometries. Formed by polymer and porous networks”,by R. A. M. Hikmet, edited by G. P. Crawford, S. Zumer, published byTaylor & Francis, London, 1996. Such a network-enforced liquid crystallayer may, for example, be created in-situ in that a previously appliedreactive mixture is irradiated with UV-light, as is described in thisreference.

When no potential difference is applied between the first and secondtransparent electrodes 71 and 73, the liquid crystal molecules of thefirst information layer 72 are randomly directed, so that the firstinformation layer 72 is substantially transparent at the wavelength 1.When a suitable potential difference is applied between the first andsecond transparent electrodes 71 and 73, the liquid crystal molecules ofthe first information layer 72 will turn towards the direction of theelectric field created by said potential difference, which results inthe first information layer 72 becoming absorbent and/or reflective atthe wavelength 1. This is a consequence of a change in index ofrefraction, which results from the re-orientation of the liquid crystalmolecules of the first information layer 72.

The molecules used in accordance with the invention may alternatively bemolecules comprising a charged substituent which turn towards thedirection of a current created by the potential difference appliedbetween two transparent electrodes. Examples of such molecules areionomers and polyelectrolytes. Polyelectrolytes or ionomers areion-containing polymers, consisting of polymeric backbones with arelatively small number of monomer units with an ionic functionalityeither as a pendant group or incorporated in the main chain. Mostly,structures with carboxylic, sulfonic, or phosphoric acids can be used,which are partly or fully neutralized with cations. These materials aredescribed, for example, in “Ionic Polymers”, by L. Holliday, AppliedScience Publishers, London, 1975. Particular examples of these materialsare, for example, poly(2-acrylamido-2-methylpropanesulphonic acid),poly(ethylene sulphonic acid), poly(styrene sulphonic acid), and zinc orsodium salts of copolymers such as poly(ethylene-co-methylacrylic acid).

Optionally, these polyelectrolytes or ionomers may be modified so as tocomprise mesogenic units, present in the polymeric main-chain,side-chain or both. Specific examples of such liquid crystallineionomers are described, for example, in “Liquid-crystalline ionomers”,by Wilbert et al., Macromolecular Symposia (1997), 117 229-232.

Futhermore, optional additives such as solvent, co-solvent, or softeningadditives may be used along with the employed ionomers orpolyelectrolytes in order to adjust the viscosity of the informationlayer, so as to facilitate and optimise the reorientation of thematerials.

When no potential difference is applied between the first and secondtransparent electrodes 71 and 73, the molecules of the first informationlayer 72 are randomly directed, so that the first information layer 72is substantially transparent at the wavelength 1. When a suitablepotential difference is applied between the first and second transparentelectrodes 71 and 73, the molecules of the first information layer 72will turn towards the direction of the current created by said potentialdifference, which results in the first information layer 72 becomingabsorbent and/or reflective at the wavelength 1.

This direction depends on the nature of the materials used in the firstinformation layer 11. If the first information layer 11 only comprisescharged substituents, this direction is the direction of the currentcreated by said potential difference. If the information layer comprisescharged substituents containing mesogenic units, the direction dependson the nature of the liquid crystal molecules of the mesogenic units.

The following description applies to information layers comprisingliquid crystal molecules. A similar description applies to informationlayers comprising molecules with a charged substituent, optionallycontaining mesogenic units.

When the first information layer 72 is scanned for reading informationfrom this first information layer 72, a potential difference V1 isapplied between the first and second transparent electrodes 71 and 73.An electric field is thus created between the first and secondtransparent electrodes 71 and 73. As a result, the liquid crystalmolecules of the first information layer 72 turn towards the directionof this electric field, i.e. a direction substantially orthogonal to thefirst and second transparent electrodes 71 and 73. As a consequence, thefirst information layer 72 becomes absorbent and/or reflective at thewavelength 1.

The potential difference V1 is chosen such that, when it is applied, theabsorption and/or reflection of the first information layer 72 becomerelatively high at the wavelength 1. Then, once the absorption and/orreflection of the first information layer 72 have become high,information can be read from this information layer by conventionalread-out techniques.

Once the information of the first information layer 72 has been read,the second information layer 76 is scanned. First, the first informationlayer 72 is made transparent in that the potential difference V1 isremoved. The electric field between the first and second transparentelectrodes 71 and 73 disappears, the liquid crystal molecules rotateback to their initial orientations and the first information layer 72thus becomes transparent.

Then, the second information layer 76 is made absorbent and/orreflective, in that a potential difference V2 is applied between thethird and fourth transparent electrode 75 and 77. In this example, V2 isequal to V1, because the first and second information stacks comprisethe same liquid crystal molecules. If different molecules having anability to turn towards a given direction are used in the first andsecond information layers 72 and 76, V2 may differ from V1. Also if thelayer thicknesses of the information layers 72 and 76 are different, adifferent potential difference may be needed.

Once the second information layer 76 is absorbent and/or reflective,information can be read from this second information layer 76. The firstinformation layer 72 does not perturb the read-out of information,because the first information layer 72 has been made transparent. As aconsequence, it is possible to address only one information layer, whilethe rest of the information carrier is substantially transparent. Thedesired layer is addressed by application of the suitable potentialdifferences between the transparent electrodes of the individualinformation stacks.

In the above description, the liquid crystal molecules are randomlyoriented when no potential difference is applied between the first andsecond electrodes. When a potential difference is applied, they turntowards a direction, which is parallel or perpendicular to the electricfield created by the potential difference, depending on the nature ofthe liquid crystal molecules.

It should be noted that the liquid crystal molecules may also beoriented in a certain direction when no potential difference is applied,this direction being changed when a potential difference is appliedbetween the first and second electrodes. For example, the liquid crystalmolecules may be parallel to the first and second electrodes when nopotential difference is applied, assuming this orientation results in atransparent information layer. Then, when a voltage difference isapplied, the liquid crystal molecules turn towards a directionperpendicular to the first and second electrodes, in which case theconsidered information layer becomes absorbent and/or reflective.

In the latter case, the liquid crystal molecules should return to theirinitial orientation when the potential difference is removed. This maybe achieved by use of an anisotropic network for the information layer.For example, if the orientation of the liquid crystal molecules isplanar when no potential difference is applied, i.e. parallel to thefirst and second electrodes, a planarly oriented anisotropic networkwill be used in combination with liquid crystal molecules having apositive dielectric anisotropy. If the orientation of the liquid crystalmolecules is homeotropic when no potential difference is applied, i.e.perpendicular to the first and second electrodes, a homeotropicallyoriented anisotropic network will be used in combination with liquidcrystal molecules having a negative dielectric anisotropy.

Alternatively, a chemical or mechanical modification of the first andsecond electrodes may be performed, in order to induce a preferredorientation of the liquid crystal alignment when no voltage is applied.

Alternatively, additional alignment layers that enclose the informationlayer may be used. An additional information layer is placed between anelectrode and the information layer of an information stack. Bothalignment layers are preferred, although it is also possible to use onlyone of these alignment layers.

Alignment layers typically used for the construction of conventionalliquid crystal displays may be used here, such as rubbed polyimidealignment layers, or photoalignment layers, such as coumarin derivativesor cinnamate derivatives. These layers may again be deposited byconventional processing techniques, such as spin coating or dip coating.Depending on the type of alignment layer, subsequent rubbing is requiredor a brief UV-exposure, to induce the desired orientation. The usedalignment layers enclosing the information layer are preferably thesame, but may also be different. A benefit of the use of polyimides istheir outstanding temperature stability, which is well above the typicaldegradation temperatures that are commonly observed for the majority oforganic polymers.

FIG. 8 a shows a second ROM information carrier in accordance with asecond embodiment of the invention. In this Figure, the numbersidentical to numbers of FIG. 7 stand for the same entities. Thisinformation carrier comprises a first, a second, a third and a fourthtransparent electrode 71, 73, 75 and 77, a first and a secondinformation layer 72 and 76 and a spacer layer 74. The first transparentelectrode 71, the first information layer 72 and the second transparentelectrode 73 form a first information stack; the third transparentelectrode 75, the second information layer 76 and the fourth transparentelectrode 77 form a second information stack. The two information stacksare separated by the spacer layer 74.

The information strip rolled up so as to obtain the information carrierof FIG. 8 a comprises a first transparent electrode, an informationlayer, a second transparent electrode and a spacer layer. The firsttransparent electrode is patterned by conventional techniques such asembossing, and the information layer is deposited on the patternedtransparent electrode by conventional techniques such as vapourdeposition.

In order to address the first and the second information layers 72 and76, the potential differences are applied between the first and secondtransparent electrodes 71 and 73, and the third and fourth transparentelectrodes 75 and 77, respectively.

FIG. 8 b shows a third ROM information carrier in accordance with asecond embodiment of the invention. This information carrier comprises afirst, a second and a third transparent electrode 81, 83 and 85, and afirst and a second information layer 82 and 84. The first transparentelectrode 81, the first information layer 82 and the second transparentelectrode 83 form a first information stack; the second transparentelectrode 83, the second information layer 84 and the third transparentelectrode 85 form a second information stack.

The information strip rolled up so as to obtain the information carrierof FIG. 8 b comprises an information layer and a transparent electrode.The transparent electrode comprises an adhesive material. In this case,an information stack does not correspond to a part of the informationstrip corresponding to a single turn of the winding process, but is thecombination of two parts of the information strip each corresponding toa single turn of the winding process.

In order to address the first and the second information layers 82 and84, the potential differences are applied between the first and secondtransparent electrodes 81 and 83, and the second and third transparentelectrodes 83 and 85, respectively.

FIG. 9 shows a WORM (Write Once Read Many) information carrier inaccordance with a first embodiment of the invention. This informationcarrier comprises a first information layer 91, a first electrolytelayer 92, a first counter electrode 93, a spacer layer 94 a secondinformation layer 95, a second electrolyte layer 96 and a second counterelectrode 97. The first information layer 91, the first electrolytelayer 92 and the first counter electrode 93 form a first informationstack; the second information layer 95, the second electrolyte layer 96and the second counter electrode 97 form a second information stack. Thetwo information stacks are separated by the spacer layer 94.

The information strip rolled up so as to obtain the information carrierof FIG. 9 comprises a counter electrode, an electrolyte layer, aninformation layer and a spacer layer. The information layer comprises anelectrochromic material having an ability to take up or releaseelectrons, which ability can be locally reduced by means of the opticalbeam at the wavelength 1. In order to locally reduce the ability to takeup or release electrons of the electrochromic materials, a relativelyhigh power of the optical beam is required. The high power is absorbedin the material and changes its material properties, for example bymelting, annealing, photochemical reactions, thermal damaging ordeterioration. This relatively high power is used during writing ofinformation on the information carrier, whereas a lower power is usedduring reading, the latter being insufficient for reducing the abilityto take up or release electrons of the electrochromic materials.

In order to write information on the first information layer 91, theoptical beam having the relatively high power is focused on the firstinformation layer 91, in order to locally reduce the ability to take upor release electrons of the electrochromic material, for writing marks.In FIG. 9, the marks where the ability to take up or release electronsof the electrochromic material has been reduced are represented bydotted lines. The depth of the marks in the information layers can bechosen in that the power of the optical beam is varied, or the timeduring which the optical beam is focused on a mark is varied. Havingdifferent mark depths allows multilevel recording. In single-levelrecording, typically two reflection states or levels are used, whereasin the case of multi-level recording, more reflection levels are definedto represent data.

In order to write information on the second information layer 95, theoptical beam having the relatively high power is focused on the secondinformation layer 95, in order to locally reduce the ability to take upor release electrons of the electrochromic material, for writing marks.

The information layer on which information is to be written may be madeabsorbent before the relatively high power optical beam is focused onit. This improves absorption of the relatively high power optical beam,which increases the reduction of the ability to take up or releaseelectrons of the electrochromic material.

In order to read information from the first information layer 91, thisfirst information layer 91 is made absorbent at the wavelength 1, inthat a suitable voltage V1 is applied between the first informationlayer 91 and the first counter electrode 93. The first information layer91 becomes absorbent, except where marks have been written, because theability to take up or release electrons of these marks is too small forallowing a reduction of the electrochromic material of these marks.Hence, the difference in absorption and reflection between the marks andthe non-marked areas in the first information layer 91 is used forreading information from the first information layer 91.

In order to read information from the second information layer 95, thefirst information layer 91 is made transparent at the wavelength 1, inthat a reverse voltage −V1 is applied between the first informationlayer 91 and the first counter electrode 93. The entire firstinformation layer 91, including the marks, is made transparent thereby.Hence, the first information layer 91 does not perturb the scanning ofthe second information layer 95. Then, the second information layer 95is made absorbent at the wavelength 1, in that a suitable voltage V2,equal to V1, is applied between the second information layer 95 and thesecond counter electrode 97. The second information layer 95 becomesabsorbent, except where marks have been written. Information can then beread from the second information layer 95.

It is important to note that information layers with electrochromicmaterial having an ability to take up or release electrons which can belocally reduced by means of the optical beam at the wavelength 1 may beused in cooperation with additional electrodes, such as described withreference to FIGS. 5 a to 5 c. It should also be noted that theseinformation layers may also be used in information carriers such asdescribed in FIG. 6 a to 6 c, where an information layer serves ascounter electrode for another information layer.

It should also be noted that information layers with electrochromicmaterial having an ability to take up or release electrons which can belocally reduced by means of the optical beam may further comprise afluorescent material. In this case, the electrochromic material and thefluorescent material may be the same material. Examples of fluorescentelectrochromic materials are aminonaphtylethenylpyridinium-dyes,RH-dyes, carbocyanine derivatives and rhodamine derivatives.

Alternatively, information layers may be used, which comprise anelectrochromic material and a fluorescent material having an ability toemit light by fluorescence, which ability can be locally reduced bymeans of the optical beam. In this case, in order to write informationon the first information layer 91, the optical beam with wavelength 11,having the relatively high power, is focused on the first informationlayer 91, in order to locally reduce the ability to emit light byfluorescence of the fluorescent material, for writing marks. The sameprocess applies for writing information on the second information layer95.

The information layer on which information is to be written may be madeabsorbent before the relatively high power optical beam is focusedthereon. This improves absorption of the relatively high power opticalbeam, which increases the reduction of the ability to emit light byfluorescence of the fluorescent material.

In order to read information from the first information layer 91, thisfirst information layer 91 is made absorbent at the wavelength 11, inthat a suitable voltage V1 is applied between the first informationlayer 91 and the first counter electrode 93. The first information layer91 becomes absorbent, but a fluorescence signal is generated only whenthe optical beam is focused on a non-marked area. This property is usedfor reading information from the first information layer 91.

It is important to note that the first and second information layers 91and 95 may comprise an electrochromic material having an ability to takeup or release electrons which can be locally reduced by means of theoptical beam at the wavelength 11, and a fluorescent material having anability to emit light by fluorescence, which can be locally reduced bymeans of the optical beam at the wavelength 11. During writing, therelatively high power optical beam is used for locally reducing theability to take up or release electrons of the electrochromic materialand the ability to emit light by fluorescence of the fluorescentmaterial.

FIG. 10 a shows a first WORM information carrier in accordance with asecond embodiment of the invention. This information carrier comprises afirst, a second, a third and a fourth transparent electrode 101, 103,105 and 107, a first and a second information layer 102 and 106 and aspacer layer 104. The first transparent electrode 101, the firstinformation layer 102 and the second transparent electrode 103 form afirst information stack; the third transparent electrode 105, the secondinformation layer 106 and the fourth transparent electrode 107 form asecond information stack. The two information stacks are separated bythe spacer layer 104.

The information strip rolled up so as to obtain the information carrierof FIG. 10 a comprises a first transparent electrode, an informationlayer, a second transparent electrode and a spacer layer. Theinformation layer comprises molecules intended to turn towards adirection substantially orthogonal to the first and second transparentelectrodes when a suitable potential difference is applied between thefirst and second transparent electrodes. The first transparent electrodehas an electrical conductance which can be locally reduced by means ofthe optical beam at the wavelength 1. In order to locally reduce theelectrical conductance of the first transparent electrode, a relativelyhigh power of the optical beam is required. The high power is absorbedin the material and changes the material properties thereof, for exampleby melting, annealing, photochemical reactions, thermal damaging ordeterioration. This relatively high power is used during writing ofinformation on the information carrier, whereas a lower power is usedduring reading, which power is not able to reduce the electricalconductance of the first transparent electrode.

In order to write information on the first information layer 102, theoptical beam having the relatively high power is focused on the firsttransparent electrode 101, in order to locally reduce the electricalconductance of this first transparent electrode 101, for writing marks.In FIG. 3 a, the marks where the electrical conductance of the firsttransparent electrode 101 has been reduced are represented by dottedlines.

In order to write information on the second information layer 106, theoptical beam having the relatively high power is focused on the thirdtransparent electrode 105, in order to locally reduce the electricalconductance of this third transparent electrode 105.

In order to read information from the first information layer 102, asuitable voltage V1 is applied between the first transparent electrode101 and the second transparent electrode 103. An electric field iscreated between the first and second transparent electrodes 101 and 103,except where marks have been written, because the electrical conductanceof these marks is to small for generating an electric field. Hence, theliquid crystal molecules of the first information layer 102 aresubjected to an electric field, except in the parts located under themarks written in the first transparent electrode 101. As a consequence,the first information layer 102 becomes absorbent and/or reflective,except in the parts located under the written marks.

The difference in absorption and reflection between the parts under themarks and the parts under the non-marked areas is thus used for readinginformation from the first information layer 102.

In order to read information from the second information layer 106, thefirst information layer 102 is made transparent at the wavelength 1, inthat the potential difference V1 is removed. The entire firstinformation layer 102 is made transparent thereby. Hence, the firstinformation layer 102 does not perturb the scanning of the secondinformation layer 106. Then, the second information layer 106 is madeabsorbent and/or reflective at the wavelength 1, in that a suitablevoltage V2, equal to V1, is applied between the third transparentelectrode 105 and the fourth transparent electrode 107. The secondinformation layer 106 becomes absorbent and/or reflective, except in theparts located under the marks written in the third transparent electrode105. Information can then be read from the second information layer 106.

It should be noted that the thicknesses of the layers as compared withthe mark width represented in FIG. 10 a do not necessarily correspond toreality. Actually, it is advantageous that the thickness of aninformation layer is smaller than the width of a mark. Actually, if thethickness of an information layer is greater than the width of a mark,an electric field may be created even in parts located under marks.Hence, the parts where the liquid crystal molecules are subjected to anelectric field may be larger than desired, thus reducing the datacapacity of such an information carrier. For optical recording, themarks are typically larger than 500 nanometres. As a consequence, athickness of the information layer smaller than 300 nanometres ispreferred, in order to avoid the creation of an electric field in a partlocated under a written mark.

It should also be noted that the information layer preferably has adecomposition temperature which is higher than the temperature at whichthe electrical conductance of the first transparent electrode isreduced. Actually, even if the optical beam is not directly focused onthe information layer during writing, the information layer reaches atemperature which is not far from the temperature of the transparentelectrode in which marks are written.

However, an information layer having a decomposition temperature lowerthan the temperature at which the electrical conductance of the firsttransparent electrode is reduced may be used in a WORM informationcarrier in accordance with a second embodiment of the invention, asshown in FIG. 10 b. In FIG. 10 b, the information carrier furthercomprises a first and a second thermal insulation layer 108 and 109,which are placed between the first transparent electrode 101 and thefirst information layer 102, and between the third transparent electrode105 and the second information layer 106, respectively.

The information strip rolled up so as to obtain the information carrierof FIG. 10 b comprises a first transparent electrode, an informationlayer, a thermal insulation layer, a second transparent electrode and aspacer layer.

The first and second thermal insulation layers 108 and 109 are chosen soas to be transparent at the wavelength 1, and to have a decompositiontemperature higher than the temperature at which the electricalconductance of the first and third transparent electrodes 101 and 105 isreduced. For example, a ZnS—SiO2 layer may be used as the thermalinsulation layer, as well as high-temperature resistant polymers, suchas polyimides, polyetherimides, polyesterimides, polyamidimides,polyamides, polymetylpentene, polyetheretherketone, andpolyethersulfone. The first and second thermal insulation layers 108 and109 have a relatively low thermal conductivity. As a consequence, thetemperature of the first and second information layers 102 and 106during writing is lower than the temperature of the first and thirdtransparent electrodes 101 and 105. Hence, the first and secondinformation layers 102 and 106 can have a relatively low decompositiontemperature.

FIG. 10 c shows a third WORM information carrier in accordance with asecond embodiment of the invention. Compared with the WORM informationcarrier of FIG. 10 a, this information carrier further comprises afirst, a second, a third and a fourth additional electrode 1010 to 1013.The additional electrodes serve to overcome the local increase inelectrical resistance when the first and third electrodes 101 and 105,in which marks are written, are partially degraded. Organic conductingpolymers with a high degradation temperature or inorganic layers such asITO (Indium-Tin-Oxide) may be used as additional electrodes.

FIG. 10 d shows a fourth WORM information carrier in accordance with asecond embodiment of the invention. This information carrier comprises afirst, a second, a third and a fourth electrode 101, 103, 105 and 107, afirst and a second information layer 102 and 106 and a spacer layer 104.The first electrode 101, the first information layer 102 and the secondelectrode 103 form a first information stack; the third electrode 105,the second information layer 106 and the fourth electrode 107 form asecond information stack. The two information stacks are separated bythe spacer layer 104.

The information layers can be locally degraded, e.g. annealed, altered,molten, fixed, degraded, crystalized, or photochemically deteriorated bymeans of an optical beam. In order to locally degrade the first andsecond information layers 102 and 106, a relatively high power of theoptical beam is required. The high power is absorbed in the material andchanges its material properties, for example by melting, annealing,photochemical reactions, thermal damaging or deterioration. Thisrelatively high power is used during writing of information on theinformation carrier, whereas a lower power is used during reading, whichlower power is not able to degrade or alter the first and secondinformation layers 102 and 106.

A local degradation of an information layer of an information stack hasthe result that the molecules in a degraded area lose their ability torotate when a potential difference is applied between the first andsecond electrodes of this information stack. Hence, degraded areasremain transparent, whatever the potential difference applied betweenthe first and second electrodes of this information stack.

In order to write information on the first information layer 102, theoptical beam having the relatively high power is focused on the firstinformation layer 102, in order to locally degrade this firstinformation layer 102, for writing marks. In FIG. 10 d, the marks wherethe first information layer 102 has been degraded are represented bydotted lines. The depth of the marks in the information layers can bechosen in that the power of the optical beam is varied, or the timeduring which the optical beam is focused on a mark is varied. Havingdifferent mark depths allows multilevel recording. In single-levelrecording, typically two reflection states or levels are used, whereasmore reflection levels are defined to represent data in the case ofmulti-level recording.

In order to write information on the second information layer 106, theoptical beam having the relatively high power is focused on the secondinformation layer 106, in order to locally degrade this secondinformation layer 106, for writing marks.

The information layer on which information is to be written may be madeabsorbent before the relatively high power optical beam is focusedthereon. This improves absorption of the relatively high power opticalbeam, which increases the local degradation of the information layer.

In order to read information from the first information layer 102, thisfirst information layer 102 is made absorbent at the wavelength 1, inthat a suitable voltage V1 is applied between the first electrode 101and the second electrode 103. The first information layer 102 becomesabsorbent and or/reflective, except where marks have been written,because the molecules of these marks cannot rotate. Hence, thedifference in absorption and/or reflection between the marks and thenon-marked areas in the first information layer 102 is used for readinginformation from the first information layer 102.

In order to read information from the second information layer 106, thefirst information layer 102 is made transparent at the wavelength 1, inthat the potential difference V1 is applied between the first electrode101 and the second electrode 103. The entire first information layer102, including the marks, is made transparent thereby. Hence, the firstinformation layer 102 does not perturb the scanning of the secondinformation layer 106. Then, the second information layer 106 is madeabsorbent and/or reflective at the wavelength 1, in that a suitablevoltage V2, equal to V1, is applied between the third electrode 105 andthe fourth electrode 107. The second information layer 106 becomesabsorbent and/or reflective, except where marks have been written.Information can then be read from the second information layer 106.

FIG. 11 a shows a first RW (ReWritable) information carrier inaccordance with a first embodiment of the invention. This informationcarrier comprises a first information layer 111, a first electrolytelayer 112, a first counter electrode 113, a spacer layer 114 a secondinformation layer 115, a second electrolyte layer 116 and a secondcounter electrode 117. The first information layer 111, the firstelectrolyte layer 112 and the first counter electrode 113 form a firstinformation stack; the second information layer 115, the secondelectrolyte layer 116 and the second counter electrode 117 form a secondinformation stack. The two information stacks are separated by thespacer layer 114.

The information strip rolled up so as to obtain the information carrierof FIG. 11 a comprises a counter electrode, an electrolyte layer and aninformation layer. The information layer comprises an electrochromicmaterial. The information layer may also comprise a fluorescentmaterial. The electrolyte layer has a temperature-dependent mobilitythreshold. This means that below this threshold the mobility of ionswithin this electrolyte layer is low, whereas ion mobility is high abovethis threshold. Examples of such electrolyte layers are a polymericmatrix having a suitable glass transition, non-covalently bondedaggregates that show a suitable temperature dependent equilibriumbetween an aggregated and a free form, and a polymeric matrix having arelatively strong temperature-dependent viscosity.

In order to write a mark on the first information layer 111, the opticalbeam is focused on this mark. The electrolyte layer under this mark isheated, and the temperature of the electrolyte layer under this markexceeds the mobility threshold. A suitable potential difference V1 isapplied between the first information layer 111 and the first counterelectrode 113. As the ion mobility is low where the optical beam is notfocused, the electrochromic process takes place only where the ionmobility is high, i.e. where a mark is to be written. As a consequence,the first information layer 111 becomes absorbent only where the opticalbeam is focused, and a mark is written where this optical beam isfocused. Then the optical beam is focused on a location where anothermark is to be written on the first information layer 111. When thepotential difference V1 is subsequently cut, the written marks remainabsorbent, because of the bistability of the electrochromic material.The same process is repeated in order to write marks on the secondinformation layer 115.

The electrolyte layers are chosen so as to have a decompositiontemperature which is lower than the temperature-dependent mobilitythreshold. In that case, the information layers are not degraded duringwriting, which means that the writing process is reversible.

In order to read information from the first information layer 111, theoptical beam is focused on this information layer, and the difference inabsorption between the marks and the non-marked area is used forread-out. No potential difference is needed between the firstinformation layer 111 and the first counter electrode 113, as the marksremain absorbent without any potential difference being applied. Thesame process is repeated in order to read information from the secondinformation layer 115.

The information written on the information layers of this informationcarrier can be erased, and information can be rewritten on theseinformation layers. In order to erase information written from the firstinformation layer 111, this first information layer 111 is scanned by arelatively high power optical beam. The first electrolyte layer 112 isheated, and the temperature of the first electrolyte layer 112 exceedsthe mobility threshold. A potential difference −V1 is applied betweenthe first information layer 111 and the first counter electrode 113. Asa consequence, the written marks become oxidized and hence transparent.The whole first information layer 111 thus becomes transparent, andmarks can then be rewritten on this first information layer 111, asdescribed above. The same process is repeated in order to eraseinformation written on the second information layer 115.

It is important to note that it is possible to design a WORM informationcarrier with the information carrier of FIG. 11 a, for example, by useof an electrochromic material which exhibits an irreversible transition,i.e. which cannot be reduced once it has been oxidized, or vice-versa.Examples of electrochromic materials which exhibit an irreversibletransition are methylene red, methylene orange and erioglaucine. It isalso possible to prevent the user from applying a reverse potentialdifference, so that the written data cannot be erased. Such a limitationmay be included, for example, in a lead-in part of the informationcarrier.

In the example described above, the first information layer 111interferes with the read-out of the second information layer 115,because it comprises absorbent marks, which interact with the opticalbeam. Actually, in order to enable read-out of information written onthe information layers, the absorption of the marks has to be relativelyhigh. For example, an absorption of 20 percent is required for thewritten marks. For a filling ratio of 0.25, this leads to an absorptionof an information layer of about 5 percent. The filling ratio is theratio between the marks and the non-marked area. If the informationcarrier comprises a high number of information layers, the read-out ofthe deepest information layers is perturbed by the presence of theinformation layers located above the deepest layers. As a consequence,the number of layers is limited to about 20 in this case.

In order to increase the number of layers of such a RW informationcarrier, the information layers further comprise a thermochromicmaterial having temperature-dependent optical properties at thewavelength of the optical beam.

In this case, information is written as described above, but theelectrochromic material and the potential differences are chosen suchthat the absorption of the written marks is relatively low, for example2 percent. In order to read information from the first information layer111, the optical beam is focused on this information layer 111. As thewritten marks have a non-zero absorption, the optical beam is absorbed,and the written marks of the first information layer 111 are heated. Thetemperature of the written marks reaches a threshold above which theabsorption of the thermochromic material at the wavelength 1 becomesrelatively high. Hence, the absorption of the written marks becomessufficiently high to enable read-out of information from the firstinformation layer 111. The same process is repeated for readinginformation from the second information layer 115. During read-out ofinformation from the second information layer 115, the optical beam isfocused on the second information layer 115. Hence, the written marks ofthe first information layer 111 are not heated, and the absorption ofthese written marks remains relatively low. As a consequence, read-outof the second information layer 115 is much less perturbed by the firstinformation layer 111, if the information layers comprise athermochromic material. The number of information layers may thus beincreased with the use of a thermochromic material.

The thermochromic material may be mixed with the electrochromic materialin the information layer. It is also possible to add a layer in eachinformation stack, which layer comprises a thermochromic material and isadjacent to the layer comprising the electrochromic material. In thiscase, the information layer is the combination of the layer comprisingthe electrochromic material and the layer comprising the thermochromicmaterial.

FIG. 11 b shows a second RW information carrier in accordance with theinvention. In this Figure, numbers identical to numbers of FIG. 11 astand for the same entities. This information carrier further comprisesa first photoconductive layer 118, a first working electrode 1100, asecond photoconductive layer 119 and a second working electrode 1101.The first working electrode 1100 and the first photoconductive layerbelong to the first information stack, the second working electrode 1101and the second photoconductive layer 119 belong to the secondinformation stack. The first and second working electrodes 1100 and 1101are chosen to be transparent at the wavelength 1.

The information strip rolled up so as to obtain the information carrierof FIG. 11 a comprises a counter electrode, an electrolyte layer, aninformation layer, a photoconductive layer and a working electrode. Aphotoconductive layer allows a transfer of electrons between the workingelectrode and the information layer of its information stack, whenilluminated at the wavelength of the optical beam.

In the information carrier of FIG. 11 a, writing of a mark requiresfocusing of the optical beam on this mark during a relatively long time.During this relatively long time, the heat generated by the optical beamcan diffuse into the electrolyte layer, thus leading to a larger markthan desired, because the ion mobility of the electrolyte layer isincreased to a larger area than desired. As a consequence, onlyrelatively large marks can be written, which leads to a relatively lowdata capacity per information layer.

In order to solve this problem, each information stack comprises aphotoconductive layer, which allows a transfer of electrons between theworking electrode and the information layer of its information stack,only when it is illuminated at the wavelength 1.

In order to write a mark on the first information layer 111, the opticalbeam is focused on this mark. As a consequence, only the part locatedabove this mark is illuminated at the wavelength 1. Hence, theelectrochromic process can only take place in this mark, because theabsorption of electrons is enabled only in this mark. The electrolytelayer under this mark is heated, and the temperature of the electrolytelayer under this mark exceeds the mobility threshold. A suitablepotential difference V1 is applied between the first working electrode1100 and the first counter electrode 113. As a consequence, the firstinformation layer 111 becomes absorbent only where the optical beam isfocused, and a mark is written where this optical beam is focused. Thesame process is repeated for writing marks on the second informationlayer 115.

FIG. 12 shows a structure of an unwritten information layer in a RWinformation carrier in accordance with a second embodiment of theinvention. FIG. 12 only shows one information stack of the informationcarrier, the other information stacks being similar. This informationstack comprises a first and a second transparent electrode 121 and 123,and an information layer 122. The information layer comprises a matrix1221 and surface-charged colloidal particles, such as particles 1222 and1223. The surface-charged colloidal particles are represented byspheres, and comprise liquid crystal molecules, represented by shortrods. The representation by rods does not limit the use of liquidcrystals to be calamitic, but also banana-shaped or discotic liquidcrystals may be used. The matrix 1221 has a viscosity which can belocally reduced by means of the relatively high power optical beam atthe wavelength 1, in order to write information on the information layer122. During read-out of information, an optical beam having a lowerpower is used, which cannot reduce the viscosity of the matrix 1221. Thematrix 1221 is chosen to be transparent at the wavelength 1.

The matrix 1221 may consist of a material having a temperature-dependenttransition, which may be a first order transition, a second ordertransition, or a glass transition. Preferably, this transition will besituated well above ambient temperature, and well above the typicalupper limit handling temperature of the information carrier, but belowthe degradation temperature of adjacent layers within the informationcarrier. The matrix may furthermore have an inorganic nature, butpreferably has an organic nature, such as polymeric nature. Inparticular, a polymeric matrix may consist, for example, ofhomopolymers, copolymers or polymer blends. Examples of polymers havinga temperature-dependent transition, such as a glass transition, arepolystyrene and polymethylmethacrylate.

A method of obtaining liquid crystal molecules embedded in chargedcolloidal particles is known to those skilled in the art. For example,encapsulated liquid crystals are known from the display-related polymerdispersed liquid crystal (PDLC) switches, as described, for example, in“Liquid crystal dispersions”, by P. S. Drzaic, World Scientific,Singapore, 1995. However, the position of the liquid crystal droplets isfixed by the usually crosslinked matrix. The synthesis and use ofseparately encapsulated liquid crystals, or liquid crystalmicrocapsules, that can subsequently be dispersed in a matrix has beendescribed in, for example, S.-A. Cho, N.-H. Park, J.-W. Kim, K.-D. Suh,Colloids and surfaces, A: Physicochemical and engineering aspects, 196,217 (2002).

Various liquid crystal molecules may be used in an information carrieras depicted in FIG. 12. For example, liquid crystal molecules having apositive or negative dielectric anisotropy may be employed. Also, thetype of liquid crystal molecules may be chosen from, for example,calamitic, banana-shaped, and discotic types.

The information strip rolled up so as to obtain the information carrierof FIG. 12 comprises a first transparent electrode, an information layercomprising a matrix and surface-charged colloidal particles, and asecond transparent electrode.

When the information layer 122 is unwritten, the surface-chargedcolloidal particles are randomly dispersed in the matrix 1221. As isshown in FIG. 12, the positively surface-charged colloidal particles maycluster with the negatively surface-charged particles in order to formstable aggregates.

In this situation, the information layer 122 is substantiallytransparent at the wavelength 1, whatever the potential differenceapplied between the first and second transparent electrodes 121 and 123.Actually, the surface-charged particles comprising liquid crystalmolecules are colloidal, which means that the volume fraction ofsurface-charged particles with respect to the volume of the matrix 1221is relatively small. For example, this volume fraction is lower than 10percent. Preferably, this volume fraction is lower than 5%. It is alsopossible to use liquid molecules in the positively surface-chargedparticles different from those in the negatively surface-chargedparticles to enhance the contrast of the recorded information layer.

In order to write a mark on the information layer 122, the relativelyhigh power optical beam is focused on this mark. The part of the matrix1221 located under this mark is heated, and reaches a temperature atwhich its viscosity is reduced. A suitable potential difference V1 isapplied between the first and second transparent electrodes 121 and 123,which creates an electric field in the information layer 122, thusseparating the negatively charged colloidal particles from thepositively charged colloidal particles. A written information layer isthus obtained, which is shown in FIG. 13.

FIG. 13 shows the structure of a written RW (ReWritable) informationcarrier in accordance with the invention. In this Figure, numbers whichare identical to numbers of FIG. 12 stand for the same entities.

In the parts of the information layer 122 where a mark has been written,the positively surface-charged particles are captured at the surface ofthe negative transparent electrode, which is, in this case, the firsttransparent electrode 121, and the negatively surface-charged particlesare captured at the surface of the positive transparent electrode, whichis, in this case, the second transparent electrode 123. Once a mark hasbeen written, the relatively high power optical beam is not focused onthis mark. Hence, the part of the matrix 1221 located under this writtenmark cools down while the potential difference is maintained duringcooling down, and the surface-charged particles remain at the surface ofthe transparent electrode, because the viscosity of the matrix 1221prevents the transport of these surface-charged particles.

As a consequence, once information has been recorded on the informationlayer 122, this first information layer 122 comprises written parts,where surface-charged particles are captured at the surface of the firstand second transparent electrodes 121 and 123, and unwritten parts,where the surface-charged particles are randomly dispersed in the matrix1221.

In order to read information from the information layer 122, the lowpower optical beam is focused on this information layer, and a suitablepotential difference V2 is applied between the first and secondtransparent electrodes 121 and 123. The potential difference V2 maydiffer from V1. Actually, the potential difference V1 is used forenabling transport of the charged particles in the matrix 1221, whereasthe potential difference V2 is used for rotating the liquid crystalmolecules.

As explained in the description of FIG. 12, the unwritten parts of theinformation layer 122 remain transparent, even if the liquid crystalmolecules in these unwritten part are subjected to an electric field,because the volume fraction of charged particles with respect to thevolume of the matrix 1221 is relatively small. However, the writtenparts of the information layer 122 become absorbent and reflective atthe wavelength 1 when the potential difference V2 is applied between thefirst and second transparent electrodes 121 and 123, because of therelatively high concentration of liquid crystal molecules in a smallvolume, i.e. near the first transparent electrode 121, which moleculesall are turned towards the same direction. As a consequence, thedifference of absorption and/or reflection between the unwritten partsand the written parts of the information layer 122 can be used forread-out.

When another information layer of the information carrier is scanned,the information layer 122 is made transparent, in that the potentialdifference V2 is removed.

The information written on the information layers of the informationcarrier shown in FIGS. 12 and 13 can be erased, and information can berewritten on these information layers. In order to erase informationwritten on the information layer 122, this information layer 122 isscanned by a relatively high power optical beam. The matrix 1221 isheated, and the viscosity of this matrix 1221 is reduced. A reversepotential difference −V3 is applied between the first and secondtransparent electrodes 121 and 123, in order to enable transport of thecharged colloidal particles in a direction opposite to the transportdirection obtained during writing. The amplitude of the potentialdifference −V3, and the time during which the reverse potential −V3 isapplied between the first and second transparent electrodes 121 and 123,are chosen so as to obtain an information layer 122 as described in FIG.12, in which the surface-charged colloidal particles are randomlydispersed in the matrix 1221. Marks can then be rewritten on thisinformation layer 122, as described above.

It is important to note that it is possible to design a WORM informationcarrier with the information carrier of FIGS. 12 and 13. This ispossible, for example, in that the user is prevented from applying areverse potential difference, so that the written data cannot be erased.Such a limitation may be included, for example, in a lead-in area of theinformation carrier.

It should also be noted that multi-level recording is possible in aninformation carrier as depicted in FIGS. 12 and 13. By use of differenttimes during which the potential difference V1 is applied between thefirst and second electrodes 121 and 123, different concentrations ofpositively surface-charged particles captured at the surface of thenegative electrode 121 and negatively surface-charged particles capturedat the surface of the positive electrode 43 can be obtained.

FIG. 14 shows an optical device in accordance with the invention. Suchan optical device comprises a radiation source 1401 for producing anoptical beam 1402, a collimator lens 1403, a beam splitter 1404, a firstcorrecting lens 1405, a second correcting lens 1406, a mirror 1407, anobjective lens 1408, a servo lens 1409 for servo and data detection,detecting means 1410, measuring means 1411 and a controller 1412. Thisoptical device is intended for scanning an information carrier 1420 inaccordance with the invention. The information carrier 1420 comprisesthree information stacks and six contacts C1 to C6.

The optical device comprises a cavity or receptacle, whose shape isarranged for receiving the information carrier 1420. The mirror 1407 andthe objective lens 1408 are mounted so that they can rotate togetherinside the cavity. The mirror 1407 and the objective lens 1408 arefocusing means. When the information carrier 1420 is inserted into theoptical scanning device, the focusing means are placed inside thecentral hole of the information carrier 1420.

During a scanning operation, which may be a writing operation or areading operation, the information carrier 1420 is scanned by theoptical beam 1402 produced by the radiation source 1401. The opticalbeam 1402 is focused on an information layer of the information carrier1420, by means of the collimator lens 1403, the first and secondcorrecting lenses 1405 and 1406, the mirror 1407 and the objective lens1408.

During a scanning operation, a focus error signal or a tracking errorsignal may be detected, corresponding to a positioning error of theoptical beam 1402 on the information layer. This focus error signal andthe tracking error signal may be used in order to correct the axialposition of the first and second correcting lenses 1405 and 1406, so asto compensate for a focus error or a tracking error of the optical beam1402. A signal is sent to the controller 1412, which drives an actuatorin order to move the first or the second correcting lens 1405 or 1406axially or radially.

The error signals and the data written on the information layer aredetected by the detecting means 1410. The optical beam 1402, reflectedby the information carrier 1420, reaches the servo lens 1409, via theobjective lens 1408, the mirror 1407, the second correcting lens 1406,the first correcting lens 1405 and the beam splitter 1404. Thisreflected beam then reaches the detecting means 1410. If the informationstacks further comprise a fluorescent material, the detecting means 1410may comprise means for separating the fluorescence signal coming fromthe addressed layer from the fluorescence signals coming from thenon-addressed layers. For example, a confocal pinhole is arranged infront of a photodiode in order to spatially block the fluorescencesignal coming from the non-addressed layers. However, such means forseparating the fluorescence signal coming from the addressed layer fromthe fluorescence signals coming from the non-addressed layers areusually not necessary in an optical scanning device in accordance withthe invention, because it is only the addressed layer that emits lightby fluorescence in the information carriers in accordance with theinvention.

The following description applies to an information carrier 1420 asdepicted in FIGS. 3 a and 3 b, i.e. wherein an information stackcomprises an information layer and a counter electrode, the potentialdifferences being applied between the information layer and the counterelectrode. The description is similar for information carriers asdepicted in the other Figures.

In order to address an information layer of an information stack of theinformation carrier 1420, this layer is made absorbent and reflective inthat a potential difference is applied between the contacts which areelectrically connected to the information layer and the counterelectrode of this information stack. The optical device comprises meansfor applying a potential difference between two contacts. In the exampleof FIG. 14, a potential difference is applied between contacts C3 andC4, in order to address the corresponding information layer.

Once this information layer is addressed, the optical beam 1402 isfocused on this information layer, and information can be read. Theinformation carrier 1420 is scanned in a helical way. The informationlayer comprises a plurality of tracks, each track corresponding to acomplete revolution of the focusing means. Once a particular track hasbeen read, another track of the same information layer can be read inthat the information carrier 1420 is translated in an axial direction,i.e. the direction D indicated in FIG. 14. Alternatively, the focusingmeans may be translated inside the central hole of the informationcarrier 1420. In this case, the information carrier 1420 is fixed in theoptical scanning device.

In this example, only the mirror 1407 and the objective lens 1408 aremounted with rotation possibility inside the cavity of the opticalscanning device. It should be noted that other elements of the opticalscanning device may be rotationally mounted inside this cavity, forexample, the first and second correcting lenses 1405 and 1406 can bethus mounted inside said cavity.

It is important to note that, in order to scan a particular informationlayer, all the other information layers of the information carrier 1420do not necessarily need to be transparent. For example, in order to scanthe information layer adjacent to the central hole of the informationcarrier 1420, the other information layers can be absorbent, because theoptical beam 1402 does not pass through these information layers. Thisis advantageous, because in this case, when a next information layer isscanned, this next information layer is already absorbent. As aconsequence, this next information layer can be scanned without waitingfor it to become absorbent.

It should be noted that the fact that the information carrier cannotrotate, but only the focusing means, is particularly advantageous.Actually, the size and weight of the focusing means are relatively smallcompared with the size of, for example, a conventional CD or DVD. As aconsequence, the rotational speed of the focusing means can be higherthan the rotational speed of a conventional CD or DVD. Hence, the datatransfer rate is higher in an optical scanning device in accordance withthe invention than in a conventional CD or DVD player/recorder.

It should be noted that a counterweight may be included in the focusingmeans, in order to compensate for the high centrifugal force caused bythe high rotational speed.

It should be noted that in another embodiment, the signal correspondingto information written in the information carrier 1420 can be detectedin transmission by a second objective lens which is placed outside theinformation carrier 1420, and which can rotate around the informationcarrier 1420 such that the optical beam 1402 transmitted through aninformation layer reaches this second objective lens.

It should also be noted that in another embodiment, the informationcarrier 1420 may have a mirror at the back of the whole carrier, whichmirror reflects the beam transmitted through all information stacks,including the addressed one. In this case, the optical scanning deviceas shown in FIG. 14 may be used to read the data. For example, theprotective layer 17 of FIG. 1 may comprise a reflective surface.

Any reference sign in the following claims should not be construed aslimiting the claim. It will be obvious that the use of the verb “tocomprise” and its conjugations does not exclude the presence of anyother elements besides those defined in any claim. The word “a” or “an”preceding an element does not exclude the presence of a plurality ofsuch elements.

1. An information carrier (1420) for scanning information by means of anoptical beam (1402) having a wavelength, said information carriercomprising a central hole (11) and at least two information stacks rollup around said hole, wherein each information stack comprises a firstelectrode, a second electrode and a material whose optical properties atthe wavelength of the optical beam depend on a potential differenceapplied between the first and second electrodes.
 2. An informationcarrier as claimed in claim 1, said information carrier comprising anelectrolyte layer (32) between the first and the second electrode, thefirst electrode being an information layer (31) comprising anelectrochromic material, the second electrode being a counter electrode(33).
 3. An information carrier as claimed in claim 2, wherein aninformation layer serves as counter electrode for another informationlayer.
 4. An information carrier as claimed in claim 2, wherein theelectrochromic material (91) has an ability to take up or releaseelectrons, which ability can be locally reduced by means of the opticalbeam in order to write information on the information layer.
 5. Aninformation carrier as claimed in claim 2, wherein the electrolyte layer(112) has a temperature-dependent mobility threshold.
 6. An informationcarrier as claimed in claim 5, wherein the information layer furthercomprises a thermochromic material having temperature-dependent opticalproperties at the wavelength of the optical beam.
 7. An informationcarrier as claimed in claim 5, wherein an information stack furthercomprises a photoconductive layer (118) for allowing a transfer ofelectrons in the information layer when illuminated at the wavelength ofthe optical beam.
 8. An information carrier as claimed in claim 2,wherein the information layer (41) further comprises a fluorescentmaterial.
 9. An information carrier as claimed in claim 8, wherein thefluorescent material has an ability to emit light by fluorescence, whichability can be locally reduced by means of the optical beam in order towrite information on the information layer.
 10. An information carrieras claimed in claim 1, said information carrier comprising aninformation layer (72) between the first and second electrodes (71, 73),wherein the information layer comprises molecules which can be rotatedwhen a suitable potential difference is applied between the first andsecond electrodes.
 11. An information carrier as claimed in claim 10,wherein said molecules are liquid crystal molecules which can be rotatedwhen subjected to an electric field created by the potential differenceapplied between the first and second electrodes.
 12. An informationcarrier as claimed in claim 10, wherein said molecules comprise acharged substituent which can be rotated when subjected to a currentcreated by the potential difference applied between the first and secondelectrodes.
 13. An information carrier as claimed in claim 10, whereinthe first electrode (101) has an electrical conductance which can belocally reduced by means of an optical beam in order to writeinformation on the information layer.
 14. An information carrier asclaimed in claim 13, wherein the information stack further comprises athermal insulation layer (108) between the first electrode and theinformation layer.
 15. An information carrier as claimed in claim 10,wherein the information layer can be locally degraded by means of anoptical beam in order to write information on the information layer. 16.An information carrier as claimed in claim 10, wherein the informationlayer (122) comprises a matrix (1221) comprising two types ofsurface-charged colloidal particles, one with negative charge and onewith positive charge (1222, 1223), said charged colloidal particlescomprising liquid crystal molecules, said matrix having a viscositywhich can be locally reduced by means of an optical beam in order towrite information on the information layer.
 17. An optical scanningdevice for scanning an information carrier (1420) by means of an opticalbeam (1402) having a wavelength, said information carrier comprising acentral hole (11) and at least two information stacks roll up aroundsaid hole, wherein each information stack comprises a first electrode, asecond electrode and a material whose optical properties at thewavelength of the optical beam depend on a potential difference appliedbetween the first and second electrodes, said optical scanning devicecomprising means for receiving said information carrier, means (1401)for generating the optical beam, means for applying a potentialdifference between the first and the second electrode of an informationstack, means for focusing (1407, 1408) said optical beam on aninformation layer, said focusing means being mounted with rotationpossibility inside said receiving means.
 18. An optical scanning deviceas claimed in claim 17, wherein said focusing means are mounted withtranslation possibility inside said cavity.
 19. A method ofmanufacturing an information carrier, said method comprising the stepsof manufacturing an information strip comprising at least one electrodeand a material whose optical properties at the wavelength of the opticalbeam depend on a potential difference applied between two electrodes,winding said information strip around a transparent hollow element (12)and cutting the electrode at each turn in the winding step.
 20. A methodof manufacturing an information carrier as claimed in claim 19, saidmethod further comprising a step of writing information on theinformation strip.